Raman Scattering in the Inorganic Spin-Peierls Systemα′-Na1-δV2O5

Abstract

We have studied the spin-Peierls (SP) transition in α'-Na 1-δ V 2 O 5 (δ= 0, 0.01 and 0.1) by means of Raman scattering. At room temperature, we observe six A 1 and three A 2 phonon modes and a broad Raman band. Below T SP several new peaks and a new broad band appear. The new peak at 62 cm -1 originates from the SP-gap excitation. The new peak at 128 cm -1 and the new broad band between 130 and 400 cm -1 come from two magnetic excitations. The new peaks at 102, 646 and 944 cm -1 are assigned to the folded phonon modes and their Na + -ion deficiency dependence shows that the defect of Na + ion suppresses the SP transition. The polarized Raman spectra below T SP suggest that the possible crystal symmetry is C s 2 ( P n ) or C 1 1 ( P 1). The asymmetric lineshape of the 531-cm -1 peak superimposed on the electronic Raman band from the d - d transition around 600 cm -1 is interpreted in terms of the Fano resonance between the electronic continuous band and the phonon with a finite lifetime. The defects of the Na + ions reduce the Fano effect because the lifetime of the phonon and the phonon-continuum interaction are decreased.